The present invention relates to a method of controlling a workpiece to revolve at a constant peripheral speed when the workpiece is cut on a lathe, and more particularly to a method of controlling the peripheral speed of the workpiece by controlling the spindle of the lathe to rotate the workpiece at a predetermined peripheral speed before cutting begins, so that cutting can be initiated immediately after the cutting tool has been positioned.
It is general practice in machining operations to select an optimum cutting speed and to cut a workpiece at the selected speed in order to increase the service life of a cutting tool and to reduce the cutting time. The optimum cutting speed is determined by the materials of the workpiece and the cutting tool used, the depth of the cut and the like.
On lathes a revolving workpiece, which is generally cylindrical in shape, is secured by a chuck to a spindle. A cutting tool is caused to travel across the revolving workpiece in order to cut into the latter by a desired amount. The cutting tool is moved in a direction normal to the axis of the spindle (or along the X axis) until the workpiece is cut to a desired depth. Therefore, the cutting speed in such a cutting operation depends on the peripheral speed at which the workpiece revolves. The peripheral speed, V, of the workpiece can be expressed by the following equation: EQU V=2.pi.Rn (1)
where n is the number of RPM of the spindle (or RPM number) and R is the radius of the workpiece.
Since the cutting tool is fed continuously or intermittently in the direction of the X axis during cutting operations on the lathe, the diameter of the workpiece where the cutting takes place changes as the cutting progresses. With the spindle rotating at a constant speed, or therefore, the cutting speed deviates from the optimum speed at the beginning of the cutting operation as the cutting advances. To cope with this difficulty, there have been provided means, such as an X-value register, for monitoring the position of the cutting tool along the X axis (or the distance between the axis of the spindle and the cutting tool), and means for computing the speed of rotation for the spindle based on the desired peripheral speed and the content of the X-value register. Each time the cutting tool is moved by a predetermined amount or distance along the X axis, or a pulse is generated to produce movement along the X axis, .+-.1 is added to the content of the X-value register depending on the direction of movement of the cutting. This enables the X-value register to store information on the current position x of the cutting tool along the X axis. Based on the current position x and the desired peripheral speed V, a computing means carries out the following computation: EQU n=V/2.pi.x (2)
The spindle is controlled to revolve at the speed n computed from time to time under real time control. According to such a mode of control, the current position x of the cutting tool while it is cutting the workpiece is equal to the radius R of the workpiece at the point of cutting, allowing the spindle to revolve at commanded peripheral speeds at all times in order to maintain the cutting speed substantially constant during the cutting operation.
The content of the X-value register is revised by movement of the cutting tool not only for cutting, but also during rapid advanced such as movement toward a starting point under a rapid-advance command. Thus the X-value register stores information about the current position x of the cutting tool at all times, based on which the rotation of the spindle is controlled. As the distance along the X axis between the point at which the cutting tool starts its rapid advanced and the point at which such rapid advance ends (or cutting starts taking place) is increased, the RPM number of the spindle at the rapid-advance starting point becomes significantly different from the RPM number of the spindle at the cutting starting point (at which point the workpiece is required to revolve at the commanded peripheral speed). The motor which is driving the spindle may not achieve the required RPM even when the cutting tool arrives at the cutting starting point, owing to the short interval of time in which the cutting tool is rapidly advanced to the cutting starting point and to the slow responsiveness of the spindle motor. Accordingly, conventional numerical control apparatuses have required the programming of a dwell command after a rapid-advance command to inhibit the cutting operation until the spindle motor attains the predetermined RPM number after the cutting tool has been positioned. Where the cutting operation is under an external sequential control, a waiting sequence must be inserted in the sequential control. The prior numerical control apparatuses have therefore been disadvantageous in that the program or external sequence is complicated, and the cutting operation is time-consuming due to the waiting or dwell time inserted.
Accordingly, it is an object of the present invention to dispense with the programming of a dwell command or the insertion of a waiting sequence in order to thereby facilitate programming or the like, and to reduce the time required for the cutting operation by eliminating or shortening the waiting time.